This invention relates to a method for converting NOx in various industrial gaseous streams resulting from combustion processes into nitrogen for discharge to the atmosphere.
In many industrial applications such as plants for the production of electricity and the like, combustion gas streams are produced which contain nitrogen oxides (NOx) in quantities greater than are permissibly discharged to the atmosphere. In such processes the gaseous stream discharged may be treated for the removal of sulfur oxides, the oxidation of carbon monoxide to carbon dioxide, and the removal of nitrogen oxides. Such processes are well known to those skilled in the art and commercial catalysts are available for use for such purposes. Relatively well-defined standards exist for the amounts of these materials which can be discharged to the atmosphere. In one such process, the gaseous stream is contacted with a selective catalyst for the reduction of NOx in the presence of ammonia to produce nitrogen and water. Desirably the gas is contacted with the selective de-NOx catalyst at temperatures from about 575 to about 800xc2x0 F., and preferably at temperatures from about 700 to about 800xc2x0 F. Typically the gases are produced in combustion processes which may be direct fired processes where steam is produced in boiler tubes for use to drive turbines and the like or in processes where air is compressed and combusted with fuels such as natural gas to produce a hot gaseous stream which is used to drive turbines. A wide variety of other industrial processes can also produce such gases. In many instances, heat is recovered from the discharged stream, which may be at a temperature as high as 1400xc2x0 F. or higher. As the gaseous stream passes through various heat exchange zones such as heat recovery banks of tubes, the temperature is dropped progressively. The catalyst is generally positioned in a zone where the temperature will be between about 575 to about 800xc2x0 F. Such techniques are well known to those skilled in the art.
In one instance, the catalyst is supplied as cubes which may be in the neighborhood of 15 inches by 15 inches by 15 inches. The cubes are wrapped in sheet metal, which is typically stainless steel and include a plurality of stainless steel screens. A suitable porous ceramic metal oxide such as titanium dioxide is used to coat the stainless steel screens. The ceramic metal oxide includes a quantity of vanadium as vanadium oxide V2O5). The vanadium is typically present as vanadium oxide and is frequently reported as V2O5. While the quantities of vanadium oxide on the catalyst can vary widely, the criteria of primary interest with respect to the catalyst is its activity with respect to the conversion of NOx. One such catalyst is Hitachi SCR Catalyst (MSDS PS5953).
When the catalyst is spent as indicated by activity tests typically provided by the vendor of the catalyst, the catalyst must be replaced. As noted previously, the catalyst may be supplied as cubes, which are simply stacked in the flow path of the gases to provide a catalytic zone through which the gases pass. A plurality of such zones may be used by using multiple stacks of the catalyst cubes. The catalyst could also be supported in other ways if desired.
Since replacement of the catalyst is relatively expensive, attempts have been made to develop a method for regenerating the de-NOx catalyst.
According to the present invention, such catalysts may be regenerated and cleaned by a method comprising contacting the catalyst with an aqueous oxalic acid solution containing from about 1 to about 4 weight percent oxalic acid to produce a cleaned catalyst; drying the cleaned catalyst to produce a dried, cleaned catalyst; contacting the dried, cleaned catalyst with an aqueous vanadium compound solution containing from about 10 to about 100 g/L as V2O5 of at least one water soluble vanadium salt to produce a rejuvenated catalyst; and, drying the rejuvenated catalyst.
In many instances it may be desirable to test the catalyst for activity prior to cleaning and rejuvenating the catalyst.